An integrated gasification combined cycle (IGCC) is divided into two parts, that is, a gas generator for generating synthetic gas by using fossil fuel such as coal and petroleum including ASU, a gasifier, a heat recovery apparatus, solid particle and sulfur removing apparatus and the like as, and a gas turbine for generating electricity by burning thus generated synthetic gas.
In case of a gasifier for generating syngas by using fossil fuel such as coal and petroleum, for the protection of the gasifier from high heat, the inside thereof should be made of ceramic materials with excellent heat resistance. It is required to use materials with good heat resistance and wear resistance for an exchanger where gas with high temperature around 1500° C. (1250˜1600° C.) generated at the gasifier is passed through, materials with high wear resistance and corrosion resistance to gas for an impurity removing apparatus, materials with excellent corrosion resistance to sulfuric acid at low temperature for a sulfur removing apparatus, and thermal shield materials with low thermoconductivity for a gas turbine where a moisture content in waste gas is high and it shows high thermoconductivity.
In particular, among the parts of the integrated gasification combined cycle, a heat exchanger, a filter, a turbine and other iron-based metal parts should have strong resistance to high temperature and significant difference in pressure. Further, these parts are exposed to corrosive gas and numerous particles that wear off the materials and deteriorate their function, which results in significantly decreasing life span of the parts.
Therefore, there have been several studies in surface coating techniques that are capable of remarkably increasing life span of the parts by coating the surface of the metal parts with a material having high corrosion resistance and wear resistance.
As a material for the parts such a heat exchanger and a filter, stainless alloys (SS 304, SS 405, SS 410 and the like) and nickel alloys (HR 160, Incoloy 800, etc.) have been widely used. Especially, the nickel alloys are very expensive as compared with the stainless alloys. Therefore, there is a need to develop a surface coating method which employs inexpensive metal materials rather than conventionally used expensive metal materials and forms a coating layer with high corrosion resistance and wear resistance at the surface thereof, which makes it possible to use under severe corrosive environment.
In order to improve corrosion resistance and wear resistance of the metal material, there have been many studies in techniques of forming a coating layer by using metal borides such as iron boride (FeB, Fe2B), nickel boride (Ni2B, Ni3B) and cobalt boride (CoB, Co2B) at the surface of a material. Among these metal borides, because cobalt boride has many functional properties, there have been several attempts to apply it to various industrial fields.
Cobalt boride has been widely used as a catalyst in the field of hydrogen storage and fuel cell technology due to its excellent electrochemical properties, and applied to a wear- and corrosion-resistant coating layer due to its high hardness and excellent oxidation resistance. Further, there are numerous attempts to apply cobalt boride to the fields of biomedical and drug delivery system.
In addition, iron-based parts that have been used in the production of non-ferrous metals (aluminum, zinc and the like) by using a casting method react with melted non-ferrous metals, which results in forming brittle intermetallic compounds, leading to a drastic reduction in life span of the parts.
To solve these problems, Korean Patent Application Publication No. 2011-0004973 discloses a method of improving life span of a metal part used for non-ferrous metal casting, comprising forming cobalt boride at the surface of the metal part by boronizing treatment of expensive cobalt-based alloys, comprising 13˜74.4% of Co, 0.1˜3% of C, 15˜35% of Cr, 5˜30% of Mo, 0.5˜4% of Si, and 5˜15% of W.
Further, it has been reported that in order to provide wear resistance to the surface of a steel part or copper with high electroconductivity, it is electroplated with cobalt by using an electroplating method, followed by boronizing, to thereby form cobalt boride, which makes it possible to improve wear resistance of the surface.
Thus, cobalt boride has been known as a coating layer having good physical properties which is capable of enhancing corrosion resistance and wear resistance at the surface of a metal part. In order to form a cobalt boride coating layer at the surface of an iron-based part, the two-step surface treatment in which the surface of a material is plated with cobalt by using an electroplating method and subjected to boronization is required. However, such a two-step process is very complicated and its effect on the improvement in physical properties is not enough.
In particular, it is very difficult to uniformly coat the surface of a heat exchanger, a filter, a turbine and other iron-based parts having a complicated configuration among the parts of an integrated gasification combined cycle by using an electroplating method.